Intraocular pressure (IOP) pressure is important for determining existence of disorders in the eye such as, for example, glaucoma. Such disorders bring about a change in the normal intraocular pressure in the eye. Therefore, increased intraocular pressure may signal the onset of glaucoma which can damage the ocular nerve. Measure of IOP is thus employed as an important diagnostic tool. The periodic measurement of IOP pressure, and finding the pressure to be in normal range, assists in effectively ruling out the existence of glaucoma. As such, screening for glaucoma is a major concern of public health efforts.
Testing of IOP (tonometry) has become a routine diagnostic method for detecting glaucoma. Tonometry may be defined as the noninvasive measurement of intraocular pressure. There are two tonometry methods: 1) Applanation tonometry is a process where the cornea of the eye is flattened or applanated and pressure to achieve applanation is measured per corneal area applanated; and 2) Indentation tonometry wherein the cornea is indented with a shape other than a flat surface.
Applanation by pressing the tonometer against an exterior (corneal) surface of the eye results in a flattening to a given and certain area of measurement surface of the eye and then determining the amount of force required to produce the flattening or indentation. Intraocular pressure is determined at a point when a known pressure is applied and a known applanation area is attained.
Indentation tonometry is generally accomplished by non-contact (puff) tonometers. Such non-contact tonometers use a puff of air to flatten a circular area of the cornea, whereby intraocular pressure can be measured without physical contact between the tonometer element and the eye. The pressure is derived from the force of the airstream against the eye at the instant of corneal indentation. Non-contact tonometers avoid the necessity of anesthetizing the patient's cornea. But these puff tonometers are relatively expensive; they lack accuracy and are highly dependent on operator skill. They are unreliable in the presence of corneal disease or irregular corneal surface. Therefore, they are generally used only as screening devices and are not relied upon for accurate IOP measurements.
The applanation tonometry devices presently in use for measuring intraocular pressure (contact tonometers) are placed upon, and then pressed against, the cornea of the eye. The area of tonometer contacting the surface of the eye increases as the force applied increases, because the eye deforms under the pressure of the tonometer. The pushing force divided by the contact area give a pressure, (dynes per square millimeter for example), which equals the pressure in the eye. This relationship exists because of the characteristics of the eye: mainly, the fact that it is essentially an incompressible fluid contained within an elastic membrane.
Since the pressure measurement cannot be made unless the contact area is known, prior art devices pressed the tonometer onto the eye until the entire end of the tonometer was in contact with the eye. When the operator determined the tonometer-eye contact was complete, then the force scale portion of the tonometer was read. That force, divided by the tonometer-eye contact area, gives the intraocular pressure. Thus the prior art devices rely in large part on the operator's judgment that the entire end of the tonometer is in contact with the eye; i.e., the point at which to stop further application of force against the eye and to take the force reading.
One such contact tonometer is disclosed in U.S. Pat. No. 3,070,997 to Papritz, (commonly referred to as the "Goldmann applanation tonometer"), wherein an applanating prism is pressed against the eye with gradually increasing force while the operator illuminates and views the applanated corneal zone through the prism base. A sodium fluorescein solution is applied to the eye for staining lacrimal fluid, so that the applanation zone becomes readily visible. The tonometer tip is viewed through one-half of a slit lamp bimicroscope, so that at the point of applanation, the operator can align two semicircles and then remove the tonometer tip from the eye to read the resulting pressure value analogue from a force applying knob. This represents a two-step process which must be done for each eye.
The Goldman applanation tonometer relies heavily on operator judgment. Measurement of the force and reading force off a diode weighted plunger put on the eye by a split screen image. The operator dials in a force until the image on the screen lines up a certain way. Also, sensitivity of the patient to fluorescein may be a problem if the test requires introduction of that foreign agent in the eye.
Pneumatic tonometers (known as pneumotonometers) generally include an air bearing piston having a pivoting probe tip to contact the corneal surface when urged forward by gas flowing to the probe tip beneath a thin membrane to emerge through small vents. Corneal pressure against the membrane causes a seal, but as gas pressure rises pushing the membrane against the eye with increasing force, pressure becomes sufficiently high to void the seal beneath the membrane. Gas escapes through the vents and pressure stabilizes at or very near intraocular pressure. However, such devices may be inaccurate at high IOP ranges; frequent maintenance is required; there is a requirement for use of an inert gas, such as freon and the gas cylinders must be replaced at regular intervals; and such devices may be problematic for the occurence of membrane leaks.
There is a need for an improved contact tonometer which would operate with a high degree of accuracy; obviate the need for fluoroscein and the use of any gas in intraocular pressure measurement; be manufactured by relatively simpler and less expensive production; simultaneously register the force applied and corneal area contacted in a single step process; reduce or eliminate the number of moving parts; and effectively diminish the potential variance of IOP measurements which may result from operator error variance due to moving parts.